The balance wheel as is found in a horological mechanism or other precision instrument is required to oscillate isochronically providing the required inertia to the balance spring's restoring couple. For a given restoring couple of the balance spring the inertia of the balance may be found by adjusting the centre of its gyratory mass. This is the case in high quality time keepers which have a fixed active length of balance spring and therefore a fixed restoring couple at a given temperature.